The present invention relates to bonded aggregate structures used primarily for high temperature applications. More particularly, the present invention relates to materials required to protect holding vessels containing both ferrous and non-ferrous metals, molten glass, and airborne high temperature environments such as boilers and furnaces of all kinds. The present invention further relates to the composition for forming such structures and binders for use in such compositions.
Bonded aggregate compositions are a class of known materials useful for many purposes. The class includes such products as concrete repair materials, heat resistant floor materials, high temperature refractory materials, high temperature insulation materials, and fire resistant materials. Bonded aggregate compositions generally comprise a suitable aggregate (a filler which determines the structural characteristics of the compositions before and after heat up) bound by a binder such as a high temperature cement.
Conventional cements used for bonding aggregates include aluminous cements, hydraulic cements, and Portland cements. Hydraulic cements are mixtures of fine ground lime, alumina, and silica which set to a hard product upon admixture with water, the water combining chemically with the cement constituents to form a hydrate. Portland cements are particularly hydraulic cements composed of lime, alumina, silica, and iron oxide (as tetracalcium aluminoferrate), tricalcium aluminate, tricalcium silicate, and dicalcium silicate. Portland cements contain generally less than five percent alumina.
Aluminous cements, in contrast, are hydraulic cements which contain at least thirty to thirty-five percent alumina, which is usually applied by the inclusion of calcined bauxite.
The cement or binder is selected to match the particular use for which the bonded composition will be used and to match the particular aggregate employed, which is similarly chosen in view of the ultimate use of the bonded composition.
Binder systems based upon phosphates have been employed, but the use of such systems normally include phosphoric acid (or a salt thereof in the presence of water) in combination with an inorganic metal oxide such as magnesium oxide or alumina oxide. Two types of products can be formed from such systems: some experiencing high temperatures during setting while ultimately achieving a high p.s.i. strength and achieving a quick set of the product at ambient temperature (15 degrees F to 85 degrees F), and others having a controlled set by application of significant heat (200 degrees F to 500 degrees F) to bring about a final set. The major drawbacks of these systems are the short working times available for the ambient set formulations, which are typically on the order of one minute. While it is preferred in working with bonded aggregates of any type that the composition set fairly rapidly, a working time of at least three minutes and preferably a range of seven to ten minutes is most desirable. With controlled set the user is extremely limited as to where the materials can be used. Both of these compositions are very sensitive to impurity when exposed to temperatures above 2500 degrees F. This limits their use both from an economic as well as practical point of view.
Applicant has discovered that the problems of the control of the setting speed of the aggregate composition, the sensitivity of the composition to impurities and the additional costs associated with providing a variety of binders for different aggregates can all be overcome by the use of a binder system comprising at least one phosphate-providing component which is in a liquid phase at ambient temperature and pressure, and at least one dry component containing CaO, Al2O3, SiO2, and Fe2O3. In one embodiment, the dry component comprises a calcium-providing component, a magnesium-providing component, or mixtures thereof. The binder may, but does not have to contain, a dry phosphate-providing component. In one embodiment, a dry phosphate-providing component and a calcium-providing component are preferably contained in a single material such as hydrated monocalcium phosphate (Ca H2PO4 H2O). However, although these two components are contained in a single material in this embodiment, an additional calcium-providing material (such as calcium aluminate cement, calcium oxide, or mixtures of them) is preferred.
Strength and dimensional characteristics can be controlled by selectively varying the concentration and ratio of the wet and dry components. The invention permits the use of commercial or preferably agricultural grade materials having lower cost than the highly purified constituents presently required in other systems. This invention does not have as a requirement the use of technical grade components, in particular technical grade phosphoric acid, as is required in other systems.
The invention is also directed to an admixture of only the dry binder components, wherein the binder is mixed with the aggregate and the wet phosphate-providing component at a remote location or where the bound aggregate structure is to be formed.
A basis for bonding in one preferred system is believed to be the admixture of a calcium-providing component such as a calcium aluminate with a wet phosphate-providing constituent such as a dilute phosphoric acid. Applicant has found that the optional addition of a dry phosphate-providing component to the basic binder not only varies the setting time by a controllable amount, but varying the percentage strengths of the aqueous phosphates to the percentage strengths of the dry phosphates, also alters the strengths and refractory characteristics of the aggregate structures after they are heated.
The binder of the present invention can typically be employed with a variety of aggregate structures, such as refractory structures, high temperature insulation structures, ambient temperature structures such as regular floor and road applications, fire resistant structures, as well as a repair material for these high-and ambient-temperature structures. One particularly useful ambient temperature application involves the use of this system for the overall containment and neutralization of harmful liquid wastes, such as radioactive wastes. For example, the binder of the present invention can be used in conjunction with a phosphate-containing aggregate to both neutralize the waste, as well as contain it, either by hardening the waste itself or xe2x80x9cwallingxe2x80x9d it in. In this way, environmental seepage of the harmful waste is avoided.
The optional dry phosphate-providing component of the present invention preferably also contains a calcium-providing component in a single material such as monocalcium phosphate, while the wet phosphate-providing component is typically a phosphoric acid solution, preferably an agricultural grade solution. In one embodiment, an additional calcium-providing component is included in the binder, thus the additional calcium-providing component usually comprises either a calcium aluminate cement or calcium oxide or a mixture of these materials.
The dry aggregate composition in accordance with the present invention typically comprises between fifty and ninety-five percent by weight of the aggregate, one to twenty-five percent by weight of a dry component containing CaO, Al2O3, SiO2, and Fe2O3 and zero to fifty percent by weight of the dry phosphate-providing component. Such a dry aggregate composition can be admixed with about five to eighty-five percent by weight of a wet phosphate-providing component, such as a solution of commercially available phosphoric acid solution.
The present invention is thus not only directed to the bonded aggregate composition or structure, but is also directed to a dry binder comprising a mixture of the optional dry phosphate component, the wet phosphate component and the calcium component, and also to a dry aggregate composition useful for forming an aggregate structure upon the addition of a wet phosphate component. In this situation, the dry aggregate composition comprises an aggregate, an optional dry phosphate component and a dry calcium and/or magnesium component. The invention is also directed to a method of forming a bonded aggregate structure comprising the steps of mixing a wet phosphorous-providing component with the dry aggregate composition, forming the admixture into an appropriate shape, and allowing the admixture to set.
As noted, the specific proportions of components optimal for a particular purpose can readily be selected on a trial-and-error basis, but in any event are selected so as to be adequate to achieve a bonded aggregate composition of adequate strength and utility after admixture and working. It is believed, however, that the wet-to-dry ratios are governed by the structural requirements of the finished, cured aggregate structure. Generally a lower wet-to-dry ratio will be expected to result in a bonded aggregate structure of higher strengths and stability. Dilution of the aqueous phosphate while increasing the proportion of, for example, dry phosphate and calcium aluminate, is expected to yield materials having high strengths in a lower temperature range (1200 F to 2200 F), but suffering structural failure above that range. Increasing the phosphate content in the aqueous component while simultaneously decreasing the proportions of, for example, dry phosphate and calcium aluminate, is expected to yield materials having high strengths in a higher temperature range (2200 F-3100 F) and to simultaneously extend the limit of structural failure to a higher temperature.